In the last years good results have been obtained with the "analysis through synthesis" method in digital coding of a speech signal at low transmission rates. In encoders based on such analysis-synthesis methods the decoder operation is simulated already in the encoder and the synthesis result provided by each parameter combination is analyzed and the parameters representing the speech signal are selected according to which of the selectable combinations provided the best decoding result compared to the original speech signal. In the analysis-synthesis method the synthesizing parameters to be used are thus determined on the basis of the synthesized speech signal. Such a method is also called a closed system method, because the synthesis result directly controls the selection of the synthesis parameters.
In speech coding closed system search can be applied only to the most critical parameters due to the complexity of the search, e.g. to code the excitation signal in encoders using a linear prediction model. These low transmission rate speech coding methods include Multi-Pulse Excitation Coding (MPEC) and Code Excitation Linear Prediction (CELP). The realization of both the multi-pulse excitation coding and the linear code excitation coding requires an extensive calculation process and causes a high power consumption, which in practice make them difficult to realize and utilize.
With the aid of some simplifications it was recently possible to realize analysis-synthesis methods in real time using digital signal processors, but problems related to the above mentioned calculation load and the power and memory consumption make their extensive use inconvenient and in many applications prevent the use of them. Analysis-synthesis methods are explained for instance in the patent publications U.S. Pat. No. 4,472,832 and U.S. Pat. No. 4,817,157.
For an efficient coding of the excitation signal also linear predictive coding methods based on an open system have been presented, in which a part of the samples are selected directly from the analysis-filtered signal (difference signal) to be transmitted by the decoder. This method typically produces a poorer result than the feed-back method, because in this method the synthesis result is not examined at all, and the excitation sample values are not selected on the basis of the sample signal value combination providing the best synthesized signal, as is made in the above described closed system encoders. In order to obtain a low transmission rate the number of samples must be reduced or selected, and this can be made e.g. by reducing the sampling frequency of the inverse filtered signal. A method of this kind is explained e.g. in the patent publication U.S. Pat. No. 4,752,956.
The problem is to obtain good speech quality using methods where the excitation signal is selected directly from the difference signal samples. When the excitation is selected only on the basis of the difference signal, and the actual synthesis result is not used to control the formation of the excitation, then the speech signal is easily distorted during coding and its quality is lowered.
Prior art is described below with reference to the enclosed FIG. 1 showing an embodiment of the prior art solution.
FIG. 1 shows the block diagram of a prior art analysis-synthesis coding system of the CELP type. The coding in question is a code excited linear prediction coding. In the encoder the search for the excitation signal through synthesis is realized by testing all possible excitation alternatives contained in a so called code book 100, and by synthesizing in a synthesis filter 102 speech signal frames corresponding to the alternatives (in blocks of about 10 to 30 ms). The synthesized speech signal is compared with the speech signal 103 to be coded in the difference means 104, which generates a signal representing the error. The error signal can further be processed so that in the weighting block 105 some features of the human sense of hearing are taken into account in the error signal. The error calculation block 106 calculates the synthesis result obtained using each possible excitation vector contained in the code book. Thus we obtain information about the quality provided by the use of each tested excitation. The excitation vector providing the minimum error is selected to be transmitted through the control logic 101 to the decoder. To the decoder is transmitted the address of the code book memory position, where the best excitation signal contained in the code book was found.
The excitation signal used in multi-pulse excitation coding is found by a corresponding testing procedure. The procedure tests different pulse positions and amplitudes and synthesizes a speech signal corresponding to them, and further compares the synthesized speech signal with the speech signal to be coded. Contrary to the above mentioned encoder of the CELP type, the MPEC method does not examine the quality of previously formed vectors stored in the code book when the speech signal is synthesized, but the excitation vector is formed by testing different pulse positions one by one. Then we transmit to the decoder the position and the amplitude of single excitation pulses, which were selected to form the excitation.